In prior art electrical measurement devices, it is known to use switched signals having a relatively fixed frequency, in a range which may be anywhere from 15Hz up to 10KHz, having a duty cycle which is modulated. The signal is then filtered, and the end result is an analogue, DC, voltage which is buffered and used at the output of the instrument. A variety of techniques have been used in the prior art to modulate, convert, and filter the pulse-width-modulated signal. The more widely used techniques are known to incur linearity problems, however.
One such technique uses a pulse-width-modulator (PWM), producing a PWM signal which is provided to a non-inverting amplifier after input filtration of the signal. However, the non-inverting amplifier arrangement is known to suffer from problems of nonlinearity.
One method for minimizing nonlinearity is known to be the use of an inverting amplifier in the DAC. However, such circuitry is significantly more complex and requires much more stringent characteristics than required of the non-inverting amplifier.
For example, a second inversion stage is required to obtain the proper polarity for the output signal. Moreover, precision circuitry, with precision amplifiers, is required to invert the signal in measurement apparatus using sensitive null detection and calibration techniques. Such additional circuitry introduces noise into the signals, which thus adversely affects the accuracy of the measurement apparatus.
Still another disadvantage of the use of an inverting amplifier at the input to the DAC is the fact that for circuitry using an inverting amplifier the filtering circuits must be provided in the loop of the circuit, rather than at the input terminal of the amplifier. Such a filtering arrangement, particularly for a multi-pole filter, may lead to serious stability problems, which must be overcome by still further circuit modifications.
Still further, because the filter is required in the feedback loop of the inverting amplifier, the signals provided to the amplifier are high frequency signals. Thus, the amplifier is required to have precise, stable, high frequency characteristics in order to satisfy the loop gain requirements. That is, high-frequency amplifiers are required if an inverting amplifier is used to correct nonlinearity problems.
There is accordingly a need in the prior art to provide compensation for linearity occurring in input circuits providing PWM signals to DAC devices. There is more specifically a need to overcome nonlinearity problems occurring in DAC input circuits without, at the same time, creating additional difficulties in the circuit, and requiring more complex and expensive circuitry.
Another known technique for minimizing nonlinearity, which continues to use a non-inverting amplifier and avoids the difficulties associated with the use of an inverting amplifier, utilizes software control, in which the mismatched resistances of the series and shunt switches are measured. The expected nonlinearity due to such a mismatch is accordingly calculated by the software, and the measured output voltage is corrected for the presumed non-linearity by computational methods.
However, software correction is inherently not performed in real-time, and clearly requires expensive processing of the output signal prior to effective utilization thereof.
There is thus a need in the prior art for real-time linearity correction of signals provided to a DAC from a PWM via a series-shunt circuit, without requiring additional computational steps to be performed in order to estimate and/or correct the nonlinearity.